A multi-module force-measuring device, as the term is used herein, means a force-measuring device built according to a modular design, wherein at least two forces which are to be measured independently of each other are captured individually with at least two independent force-measuring modules. A multi-module weighing device is a special kind of multi-module force-measuring device, wherein the forces that are to be measured independently are represented by the respective weight forces of at least two independent weighing objects, the so-called weighing loads. The force-measuring modules are in this case referred to as weighing modules.
To measure the force that is acting on it, each of the force-measuring modules has an electromechanical force-measuring cell which functions as a converter for the quantity being measured, as it converts the input quantity, i.e. the force, into a corresponding electrical measurement signal. In a weighing module, the conversion of the measurement quantity occurs analogously by way of a so-called weighing cell which generates electrical measurement values that correspond to the weight force exerted by the weighing object.
The measurement signals of the individual force-measuring modules or individual weighing modules, as the case may be, are generally passed on to a signal-evaluating unit for their evaluation. The signal-evaluating unit is often configured as a central processing unit whereby the measurement results obtained from the measurement signals can be visually displayed, transmitted to a central computer, or directed to a system controller.
Typical applications of multi-module weighing devices are found in automated production- and test systems to weigh objects of a uniform nature. Such systems include in particular installations for the production and/or testing of small and relatively expensive components, for example in filling- and packaging machines for tablets, capsules or ampoules in the pharmaceutical industry, or in the checking of ball bearings in the machine industry. The weighing of uniform objects is a process in which a plurality of loads are being weighed individually, for example for the purpose of check-weighing, dosage-dispensing, or the filling of containers in a compact space.
Required characteristics for a multi-module force-measuring device of this kind are that the measuring modules have a high measurement accuracy, a high reproducibility and thus a high degree of stability, that they can be arranged in the most space-saving and compact way possible, and that the individual force-measuring modules are of the simplest and most cost-effective design possible.
A multi-module weighing device is described in U.S. Pat. No. 6,112,162, wherein the analog signals of the individual weighing cells are directed to a central switching device, a so-called multiplexer. Accordingly, the amplifier as well as the analog/digital converter can be used by the individual force-measuring modules during specific time intervals. However, in this arrangement the disturbance-sensitive measurement signals of the weighing cells are carried through connecting lines to the central signal-evaluating unit, where they pass through the switching device. As a consequence, disturbance effects resulting from the transmission of the signals or originating from the switching device can reduce the quality of the measurement signals. Accordingly, the achievable accuracy is limited and the device described in this reference is less suitable for applications requiring a high level of accuracy. Furthermore, this device allows measurement processes to be performed only at a relatively slow speed, because only after a measurement cycle of a first weighing cell has been completed can a subsequent measurement cycle with the next weighing cell be started.
Further, in an arrangement which is disclosed in German Patent 10 2005 025 534 B3, the electronic circuitry required for the operation of the weighing module is arranged directly below the weighing cell, i.e. in the lower part of the weighing module. The disturbances resulting from the transmission of the measurement signals can thereby be avoided. However, the drawbacks in this arrangement are the high costs resulting from the large amount of space required and from the multiple replication of the electronic circuitry, as well as the undesirable effects on the weighing cells which occur as a result of the heat developed in the electronic components. The problem with the heat generation is that the heat produced by the electronic circuits rises upward to the heat-sensitive weighing cells in accordance with the known principle of buoyancy and can thus compromise the accuracy and/or stability of the measuring device.
European published application 1 557 648 discloses a configuration, where a subordinate force measuring device (child) is connected to a superordinate force measuring device (parent) by a shared bus structure (e.g. RS485). However, the bus structure requires a suitable coordination of the communication (e.g. addresses and/or protocol), to ensure, that only one force measuring device at a time transmits data over the bus structure. This staggering produces a relative slow data transmission and the electronics required for controlling the communication leads to the previously described problems of increased heat generation and additional costs.
It is therefore the object to propose a method for transmitting the measurement values of a multi-module force-measuring device, in particular a multi-module weighing device, and to further propose a multi-module force-measuring device and a suitable force-measuring module, whereby a simple and cost-effective design and operation of the measurement device can be achieved which meets stringent requirements in regard to measurement speed, measurement accuracy, and stability.